"If a candidate practises homosexuality, or presents deep-seated homosexual tendencies, his spiritual director as well as his confessor have the duty to dissuade him in conscience from proceeding towards ordination," it says.

"Such persons in fact find themselves in a situation that presents a grave obstacle to a correct relationship with men and women."

But the paper also stresses the Church's deep respect for homosexuals, who, it says, should by no means be discriminated against.

Some Catholic theologians feel the document is not sufficiently clear, the BBC's Peter Gould says.

That it refers to "tendencies" rather than orientation "has left many people scratching their heads," Jesuit scholar Father Thomas Reese told him.

The 18-paragraph document was published with little fanfare on Tuesday morning. The Vatican is not offering further explanation.

Truth

The chairman of a Roman Catholic evangelical group in Nigeria, Godwin Ukachi, welcomed the publication as overdue.

"I think it is right for the Church to take a stand on certain issues, especially the issue of homosexuals. Here in Africa and Nigeria... we are not at home with such attitudes," he told the BBC's World Today programme.

"Something had to be done. I think they think that the Church is taking a step in the right direction."

Critics have long objected that gay seminarians might feel they have no choice but to lie about their sexual orientation.

The guidelines specifically address this issue, urging candidates for the priesthood to tell the truth.

"It would be gravely dishonest for a candidate to hide his own homosexuality," the document says.

Observers say the new rules might lead to a dramatic drop in the number of priests, especially in the West.

The guidelines, "Instruction Concerning The Criteria of Vocational Discernment Regarding Persons With Homosexual Tendencies In View Of Their Admission To Seminaries And Holy Orders", were drafted by the Vatican's Congregation for Catholic Education and approved by Pope Benedict on 31 August.

Canon law experts note that they were not issued in forma specifica, meaning the Pope has not officially invested it with his personal authority, according to the National Catholic Reporter.

That might mean there is room for further interpretation or revision.

Homosexuals had already been barred from priesthood in a 1961 document.